This research examines the primary action of Shigella dysenteriae 1 toxin as an inhibitor of eukaryotic protein biosynthesis. Two major objectives, design to reveal Shiga toxin-induced changes in ribosome structure-function relationships, are 1) to explain, in biochemical terms, the manner by which Shiga toxin enzymatically inactivates mammalian ribosomes; and 2) to define the steps of protein biosynthesis which are specifically inhibited by the toxin as a result of ribosome modification. We show that Shiga toxin is not an in vitro inhibitor of initiation of reticulocyte protein synthesis which supports existing information that the toxin is a primary inhibitor of the peptide elongation process. Changes in ribosome structure as a result of toxin action were also investigated. It was determine that Shiga toxin does not cause hydrolysis of ribosomal RNA to yield fragments larger than 10 nucleotides. Recent studies involving RNA sequencing indicate that the 3' terminal region of 5.8S ribosomal RNA remains intact following toxin inactivation of ribosomes.